1. Field of the Invention
The present invention relates to communication apparatuses for carrying out a communication via a telephone line, and more particularly to communication-apparatus using the telephone line, wherein particularly a telephone voice communication function and a facsimile function of transmitting and receiving an image signal carried out by changing over the respective functions.
2. Description of the Related Art
Most of facsimile machines which are currently prevalent are constructed so that a telephone function and a facsimile function are carried out by switching. In such facsimile machines, a telephone function circuit and a facsimile function circuit are individually provided to realize the aforementioned two functions, and a telephone line is selectively connected to either of the two function circuits by using a change-over circuit which is realized in a relay or the like thereby carrying out each kind of communication.
Normally, telephone sets with no facsimile function are designed so that the telephone sets can be operated only with a current supply from the telephone line. These normal telephone sets are generally constructed in such a manner that a dialer IC (Integrated Circuit) is used for originating a dial call while a ringer IC is used as an incoming call detector, respectively. Consequently, facsimile machines attached with the telephone function are generally designed so that the above telephone (circuit) is simply added to the facsimile function circuit.
At the time when the telephone call is made, the telephone line is connected to the telephone function circuit to carry out a call originating operation, an incoming call receiving operation and a telephone call operation. At the time of a facsimile communication, the telephone line is connected to the facsimile function circuit to carry out the call originating operation, the call receiving operation and an operation of transmitting and receiving an image signal. Since the two function circuits carry out individually a processing for the communication in this manner, it is required that each of the two circuits satisfies a predetermined Circuit Connection Technical Standard. Consequently, both the telephone function circuit and the facsimile function circuit are required for the communication and individually have the same function circuit which satisfies the Circuit Connection Technical Standard. For example, the same function circuits are provided with a pair of dial call originating circuits, incoming call detectors, circuit controllers, direct current superimposing circuits, and termination circuits. The duplication of circuitry leads to a high cost.
The Circuit Connection Technical Standard is individually set in each country so that dealers who wish to sell communication apparatuses are required to undergo a test in each of the countries to see if they can satisfy the standard. The dealers are required to pass the test and obtain an authorization for selling the apparatuses. In the case of Japan, the standard is set by the Telecommunication Terminal Examination Association. The specification content of the standard differs from one country to another. In addition, although the voice communication circuit side and the facsimile communication circuit side are common in terms of the basic specification contents such as, for example, a direct current resistor, an impedance, a dial specification, and an incoming call detection sensitivity, original specification items are respectively included for different parts at the same time.
For example, when an attempt is made to commonly use a dial call originating circuit, it happens that a telephone call can be made by using a modem used in the facsimile communication, but a dial tone cannot be confirmed with a telephone receiver. In addition, in some countries, it is technically impossible to make a telephone call by using a modem because it is provided in the Standard that the telephone receiver shall be operated with a supply of currents from a telephone line, in the event of power failures. Furthermore, with respect to the incoming call detector, it is also technically impossible to provide the incoming call detector in some other countries because it is provided in the Standard in the countries that the detectors shall be operated with a current supply from the telephone line in case of power failures.
In addition, a termination circuit (termination resistor) serves as an impedance as seen from the side of the telephone line. Signals can be transmitted with the least loss by integrating the termination circuit with an output impedance of the telephone line. Consequently, in the Circuit Connection Technical Standard in each country it is provided that the termination circuit shall be defined as an "impedance". Conventionally, the telephone function circuit and the facsimile function circuit are provided with the termination circuit, respectively. The termination circuit in the telephone function circuit incorporates a resistor and a capacitor in the primary side of the circuit. On the other hand, the termination circuit in the facsimile function circuit incorporates a resistor and a capacitor in the secondary side of the circuit via a communication transformer.
Furthermore, the facsimile machines attached with the telephone function are constructed so that a dialer IC which serves as a call originating means for a voice communication (telephone communication) is incorporated in the telephone function circuit with the result that the telephone call can be originated from the dialer IC at the time of the facsimile communication as well.
On the other hand, a facsimile machine which is exclusively used for the facsimile communication and which is not attached with the telephone function is constituted so that the dialer IC is not used. As a result, that a signal is transmitted either with a dial pulse generated from a dial pulse generator comprising a transistor and a photocoupler or with a DTMF (Dual Tone Modulation Frequency) signal from the modem.
The dialer IC is required to be provided in the telephone line of the facsimile machine attached with the telephone function. This requirement results from a problem with the voice communication circuit and the facsimile communication circuit whereby the circuit control means and the direct current superimposing circuit are independent of each other in the voice communication circuit and the facsimile communication circuit. Thus, the dial tone cannot be confirmed on the side of the telephone set when the telephone call is made with the DTMF signal from the modem in the state in which the circuit is closed with the facsimile communication circuit like in the case of the facsimile machines exclusively used for the facsimile communication.
As a technology for solving the aforementioned problem, there is available a facsimile machine described in the Japanese Unexamined Patent Publication JP-A 6-62168. This facsimile machine is designed so that an incoming call and an outgoing call path of the telephone line is changed over at the time of the voice communication and at the time of the facsimile communication. However, there arises in this arrangement a problem in that when a line length compensating circuit is provided in the telephone function, a transmitting and a receiving signal in the facsimile communication depends on a circuit current and changes with the current so that the receiving characteristics of an image signal does not become stable.
Normally, the line length compensation is a function which is incorporated in a speech IC for use in the telephone set, the IC constituting the telephone function circuit. The line length compensation is also a function of changing with the circuit current an amplifier gain in the incoming call and the outgoing call path in the telephone line, that is, between an input from a microphone and an output from the circuit and an output from the circuit and an input from the microphone. Generally, in the Circuit Connection Standard for Telephones, this function is required. On the other hand, it is desired in the Circuit Connection Standard for Facsimile Machines that the amplifier gain is fixed.